1. Field of the Invention
The invention relates in general to a source follower, and more particularly to a source follower and current feedback circuit thereof.
2. Description of the Related Art
Source follower is a widely applied circuit nowadays. As shown in FIG. 1, the source follower 100 includes a current source Is, an N-type metal oxide semiconductor (NMOS) transistor M1, a NMOS transistor M2, and a NMOS transistor Mp. A current I1 flows through the NMOS transistors M2 and Mp. Owing that NMOS transistors are influenced by channel length modulation effect, the current I1 is not stable as ideal, thereby influencing the output signal Vout. For example, when the input signal Vin varies, the output signal Vout will have the same phase change with the input signal Vin. When the output signal Vout varies, the drain voltage of the NMOS transistor M2 and thus the voltage drop between the drain and the source of the NMOS transistor M2 will change accordingly. Therefore, due to the channel length modulation effect, the current I1 flowing through the NMOS transistor M2 will not be stable. Because the current I1 change causes variation of the voltage drop (Vgs) between the gate and the source of the NMOS transistor Mp, that is, the voltage difference between the input signal Vin and the output signal Vout, the input signal Vin cannot vary synchronously with the output signal Vout, thereby resulting in signal distortion of the output signal Vout.
In order to solve the signal distortion issue, conventionally, a current mirror composed of a P-type metal oxide semiconductor (PMOS) transistor M3 and a PMOS transistor M4 are added to the original circuit to stabilize the current I2 by way of feedback and thus to prevent distortion of the output signal Vout. However, this will lower down the operational voltage of the miniaturized integrated circuit. Therefore, the voltage drop between the source and the gate of the PMOS transistor M4 as shown in FIG. 2 may cause the NMOS transistors Mp and M2 unable to operate normally and thus the source follower 200 has no function.